The present invention relates generally to a device for delivering a liquid therapeutic preparation into the body of a patient by injection into or though the patient's skin. More particularly, the invention relates to a low-profile automatic injection device that can be worn inconspicuously under the clothing of a patient to allow a liquid therapeutic preparation (such as insulin) to be administered over an extended period of time, and that incorporates a self-emptying reservoir to eliminate the need for a pump or other type of discharge device.
Various types of automatic injection devices have been developed to allow drug solutions and other liquid therapeutic preparations to be administered by untrained personnel. Generally, these devices include a reservoir that is pre-filled with the liquid therapeutic preparation, and some type of automatic needle-driving mechanism (usually of the spring-loaded type) that can be triggered by the user. Examples of such devices may be found in U.S. Pat. Nos. 4,188,950, 4,196,732, 4,258,713, 4,227,528 and 4,378,015, all to Stephen C. Wardlaw. Still further examples can be found in U.S. Pat. No. 4,214,584 to Smirnov et al., U.S. Pat. Nos. 4,894,054 and 5,527,287, both to Miskinyar, and U.S. Pat. No. 5,616,132, to Newman.
In order to start the flow of the liquid therapeutic preparation when the needle is injected, the devices disclosed in the aforementioned patents generally employ movable ampoules, pistons or other complex arrangements which are somewhat difficult to manufacture. Moreover, the design of these devices generally requires that the reservoir be positioned above the needle driving mechanism, which results in a device of considerable height. This is not necessarily a problem when the drug solution is to be injected as a bolus at one discrete time, as most of these devices are designed to do, but it is a distinct disadvantage when the drug solution is to be infused into the patient over an extended period of time. In these latter instances, the injection device may have to be held in contact with the patient's skin (e.g., by tape or an adhesive) for several hours or more, and this is difficult to achieve when the device has a large height dimension.
Another class of devices includes those which are capable of gradually infusing a liquid therapeutic preparation into the skin of a patient. In some cases, these devices are small enough (both in height and in overall size) to allow them to be "worn" by an ambulatory patient while the liquid therapeutic preparation is being infused into the patient. Examples of devices which fall in to this class include those disclosed in U.S. Pat. Nos. 4,340,048 and 4,753,651, both to Eckenhoff, U.S. Pat. No. 4,734,092, to Millerd, U.S. Pat. No. 4,781,688, to Thoma et al., U.S. Pat. No. 4,886,499, to Cirelli et al., U.S. Pat. No. 5,656,032, to Kriesel et al., and PCT Publication Nos. WO 95/13838 and WO 97/21457, both to Elan Medical Technologies, Ltd.
Unfortunately, most of the automatic infusion devices disclosed in the prior art are fairly complex in design and, as a result, cannot be made as small and inexpensive as might be desired. Generally, the complexity of these devices results from three factors. One factor is the need for a pump or other type of discharge mechanism to force the liquid therapeutic preparation to flow out of the reservoir and into the injection or infusion needle. Another factor is the need for some type of valve or flow control mechanism to cause the liquid therapeutic preparation to begin to flow at the proper time. A third factor, which applies to those devices that are designed to inject the infusion needle into the patient automatically, is the need for a suitable injection mechanism that can be triggered by the user. The structures required to perform these functions add size and complexity to the infusion device, making it larger than desired and relatively expensive to manufacture.
Accordingly, a need exists for an automatic injection device that is small and has a low-profile configuration, allowing it to be conveniently handled and worn (preferably in an inconspicuous manner under the clothing) by an ambulatory patient. A need also exists for an automatic injection device which is capable of infusing a drug solution or other liquid therapeutic preparation into the skin of a patient over an extended period of time. Finally, a need exists for an automatic injection device whose basic design allows it to be not only small and low in height, but also simple and inexpensive to manufacture.